Influence of aging on T follicular helper (Tfh) cells. The ability of the immune system to respond to vaccinations, such as that for influenza, declines with age. To begin to understand the mechanisms responsible for this decline, it is important to determine if age-related changes in CD4 T cell priming and Tfh differentiation lead to reductions in humoral responses following vaccination. To accomplish this, we have developed model systems that permit us to explore specific questions that remain unanswered in this field. Only when these questions are addressed and answered can we then begin to design strategies for overcoming them and enhancing vaccine efficacy for elderly populations. In our studies, we have used adoptive transfer models that allow us to examine the influence of aging on specific components of the immune response individually. We have made a great deal of progress in determining the influence of age on the function of Tfh cells and how this impacts the humoral response to vaccination. Tfh have been recently defined as CD4 T cells that provide cognate help for B cell responses. They express specific cell surface markers, produce specific cytokines and can be found within germinal centers (GC) following vaccination. Our studies have revealed three important points regarding Tfh and humoral immune responses with aging: (1) There are intrinsic age-related defects in Tfh function that lead to reduced B cell expansion, differentiation, IgG production and affinity maturation; (2) In addition to intrinsic age-related changes in Tfh, we have determined that there are age-related changes in the microenvironment of secondary lymphoid organs, including changes in CCL21 expression, which contributes to reduced CD4 T cell priming; (3) Antibodies (Ab) generated following vaccination of intact aged mice exhibit significantly reduced protective capacity and an altered repertoire when compared to young mice. Together, we propose that these factors contribute to reduced Tfh differentiation and the reduced ability to generate a robust response to vaccination in aged individuals, ultimately resulting in reduced protection from subsequent infection. In this proposal, we will examine the mechanisms responsible for the reduced/altered function of aged Tfh and how adjuvants can overcome these defects. Importantly, we will also examine how this impacts the generation of a protective humoral response following infiuenza vaccination. Our hypothesis is that age-related reductions in Tfh priming, which are due to both T cell intrinsic and extrinsic changes, are a major factor in the reduced humoral response to vaccinations and that enhancing this priming can improve the immune response in aged individuals.